Gas turbine engines operate with internal gas temperatures of over 1400° C. Components in the hot gas path must withstand these temperatures reliably over a long life span. In addition, these components and their connections are subject to wide thermal changes during variations in engine operation, including engine shutdowns and restarts.
A turbine section of a gas turbine engine includes rotating blades and stationary vanes enclosed in a refractory shroud assembled in part from a series of rings of refractory segments. The inner surfaces of these segments must withstand high temperatures. Ceramics are known to be useful in meeting these requirements. However, ceramic components are difficult to attach to metallic components. Ceramic material cannot be welded to metal, and ceramic-metal braze alloys can not withstand the very high temperatures found in gas turbine engines. Ceramic material differs from metal in thermal conductivity and growth, making it challenging to attach ceramic parts to metal parts in a hot and varying environment.
Ceramic matrix composite (CMC) materials typically include layers of refractory fibers in a matrix of ceramic. Fibers provide directional tensile strength and toughness that is otherwise lacking in monolithic ceramic. CMC has durability and longevity in hot environments, and it has lower mass density than competing metals, making it useful for gas turbine engine components. It is often desirable to attach CMC parts to metal via mechanical attachment methods, such as pins and bolts. However, when bores are machined in CMC for bolts or pins the fibers are cut. Stress concentrations in and around a bore from bolt or pin loading, friction, and differential thermal growth all work to degrade the bore and separate the fiber layers. Thus, attaching CMC components to metal structures with bolts or pins is a challenge.
Some CMC shroud ring segments rely on pinned holes for carrying pressure loads. Higher load carry capability is desired for such designs. 2D laminate CMC materials are made from ceramic fibers woven into cloth form. Fiber bundles (tows) in these fabrics have natural out-of-plane undulations that are essentially pre-buckled, and are sites for premature failure under compressive loads. It is known than in-plane compressive strength in CMC is directly related to interlaminar tensile strength by this phenomenon. Current oxide-based CMCs have low interlaminar tensile strength, which limits compressive strength.
Analyses show high local shear and compression stresses in pin-loaded CMC holes. Such stresses are sufficient to cause local damage in the CMC and initiate other modes of failure, such as shear tear-out. Pin-loaded hole tests also show that local damage in the contact zone can result in local delamination and subsequent propagation of damage. Microbuckling may also cause a thickness expansion around the hole.
U.S. Pat. No. 6,670,021 describes a monolithic ceramic bushing for providing a hole in a CMC structure. The bushing is locked within the CMC, so the CMC must be formed around the bushing. The geometry used for locking the bushing in the CMC requires the bushing to undergo the same final processing as the CMC, which makes ceramic the only practical choice for the bushing material.